The present invention relates to the field of thermal printing. More particularly, it relates to improvements in thermal printers of the type adapted to produce continuous tone images on a line-by-line basis.
Thermal printers of the above mentioned type are well-known. Typically, they comprise a cylindrical platen or drum for advancing a print medium, such as plain paper, past a thermal print head, and a dye-bearing donor web which is acted upon by the print head to transfer dye to the print medium. The print head usually comprises a linear array of closely spaced heating elements (resistors) which spans the transfer drum at the print zone. Each heating element can be selectively energized to raise the temperature of the heating element to the level required to cause dye to transfer to the receiver. Alternatively, the print head may comprise a linear array of electrically conductive elements which cooperate with a "resistor" donor web to thermally print image information. Such a print head is disclosed in U.S. Pat. No. 4,800,399. In either case, by electrically addressing all printing elements in the print head array simultaneously, an entire line of image information is printed at once. To produce continuous-tone images in which, for example, each picture element (pixel) of an image line exhibits any one of a multitude of different levels of gray, the duration of the current applied to each printing element is varied, either by varying the width of the current pulse or by varying the total number of current pulses within the time interval allotted to print an image line.
In continuous-tone thermal printers, it is common to continuously advance the print medium relative to the head during the line printing operation. Such movement enhances the print quality by blending the pixels of adJacent image lines together. Moreover, it prevents sticking of the heating elements to the donor web. Continuous movement of the print media is usually effected by a precision stepper motor which advances the print drum at a substantially constant rate which is sufficiently slow to allow each pixel in a line to receive the maximum gray level density (e.g. 256 levels of gray). As may be appreciated, the requirement to move the print medium at a rate no faster than that required to produce a maximum density image on each line has a limiting effect on the rate at which prints can be produced.